In the drilling process, a viscosifier is usually added to improve the viscosity of low-solid drilling fluid, to ensure the drilling fluid has high viscosity and favorable rheological property. Besides attaining a viscosity improvement effect, a viscosifier for drilling fluids often also serves as shale inhibitor (coater), filtrate reducer, and flow pattern improver, etc. For extended reach wells, high-gradient wells, horizontal wells, high-temperature and high-pressure formations, and various sensitive formations, oil-based muds exhibit advantages such as good lubrication and formation protection performance, tolerance to contaminants and solid phases, and high contamination resistance, etc. An oil-based viscosifier of drilling fluid is an important constituent of an oil-based drilling fluid, and it can improve the viscous shearing force of the oil-based drilling fluid and ensure favorable floating, cuttings carrying, and filtrate loss reduction performance of the oil-based drilling fluid. However, the researches on viscosifiers for drilling fluids made in China and foreign countries mainly focus on the domain of viscosifiers for water-based drilling fluids, and few researches on viscosifiers for oil-based drilling fluids have been made.
There are mainly kinds of viscosifiers for drilling fluids. Commonly used viscosifiers include: modified cellulose, modified guar gum, xanthan gum, and synthetic acrylamide polymers, etc., but most of them are applicable only to water-based drilling fluids. Owing to the fact that most viscosifiers for water-based drilling fluids can spread effectively and be dissolved completely in water but the polymer molecules of these viscosifiers for water-based drilling fluids can't spread in oil-based drilling fluids, especially in pure oil-based drilling fluids that don't contain water, these viscosifiers for water-based drilling fluids can't take a viscosity and shearing strength improving effect in oil-based drilling fluids. Hence, most viscosifiers for water-based drilling fluids are inapplicable to oil-based drilling fluids. At present, viscosity improvement for oil-based drilling fluids is mainly realized by virtue of the viscosity improvement effect that comes with some materials, such as organic clay and filtrate reducing polymers for oil-based drilling fluids. Sometimes, the quantity of organic clay or filtrate reducing polymer for oil-based drilling fluids has to be increased in order to attain required viscosity and shearing force in the drilling fluid, resulting in increased cost of the oil-based drilling fluid. In addition, since the temperature of the drilling fluid increases as the well depth increases, the requirement for temperature tolerance of the viscosifier is higher when the viscosifier is used in drilling fluids to be applied in deep wells. In addition, environmental friendliness is also an important aspect of concern in the use of oil-based drilling fluids. Further improvements must be made to the environmental friendliness of existing oil-based drilling fluids. Developing and using environment-friendly oil-based drilling fluid materials, such as environment-friendly viscosifiers for oil-based drilling fluids, will be beneficial for the environmental friendliness of oil-based drilling fluids.